Ronnie Biggs has been in hospital several times during recent months
Lawyers for Ronnie Biggs say they have been given permission by the High Court to challenge Justice Secretary Jack Straw's decision to refuse him parole.
Great Train Robber Biggs, 79, is being treated at Norfolk and Norwich University Hospital for pneumonia.
Earlier this month, Mr Straw refused his bid for parole.
Biggs was taken to hospital from Norwich Prison earlier this week. Doctors caring for him said he had "little hope of recovery".
A parole board had recommended Biggs be released but Mr Straw disagreed, saying Biggs remained "wholly unrepentant" for his crimes.
The latest appeal came after the criminal's son, Michael Biggs, pleaded with the government to reconsider the case, claiming doctors told him there was "not much hope".
"It's the worst he's ever been. I have never seen him this weak," he said, adding that doctors had indicated they may not resuscitate his father should his heart stop.
On the run
Biggs' lawyer Giovanni Di Stefano said in a letter to Mr Straw that the law allows for early release where a prisoner is suffering from a terminal illness and is likely to die within three months.
Biggs, from Lambeth, south London, has been moved between his prison cell and a bed at the hospital several times.
His family say he has suffered three strokes, hip, pelvis and spine fractures, and that he is guarded by three police officers despite being unable to eat, speak or walk.
Biggs was a member of a 15-strong gang which attacked the Glasgow to London mail train as it passed through Buckinghamshire in August 1963, making off with £2.6m in used banknotes.
He was given a 30-year sentence but after 15 months he escaped from Wandsworth prison in south-west London by climbing a 30ft wall and fleeing in a furniture van.
He was on the run for more than 30 years, living in Spain, Australia and Brazil, before returning to the UK voluntarily in 2001.
A Home Office spokesman said he could not discuss cases of individual prisoners.
But he added: "We can confirm that an application for the early release on compassionate grounds of a prisoner at HMP Norwich has been received by the public protection casework section in the National Offender Management Service."